The present invention relates to inkjet printing, and, more particularly, to reducing outgassing of the ink caused by the die in the pen used in inkjet printing.
Ink-jet printing is a well-established technology, involving jetting ink either thermally, via use of a heated element that expels bubbles of ink from a firing chamber upon controlled activation, or mechanically, via use of a piezoelectric transducer that expels bubbles of ink from a chamber upon controlled activation.
The composition of the inks employed in ink-jet printing has undergone considerable modification, in an effort to improve properties related to the jetting operation as well as to improve the properties of the printed ink.
Commercially-available inkjet inks have a high water content for printing on plain paper, typically on the order of 70 wt % or more water. These inks generally print well on plain paper. However, high water content ink has the disadvantage of degassing in the pen and inducing a deprime as the pen fills with air. Solutions to the outgassing issue have been found by increasing the non-aqueous solvent in the ink. However, this solution often results in inks with decreased edge acuity and optical density as the inks penetrate and wick into the paper.
One problem of particular concern in thermal printing systems (e.g., thermal inkjet printers) is a condition known as xe2x80x9coutgassingxe2x80x9d. The term xe2x80x9coutgassingxe2x80x9d basically involves the formation of gas bubbles directly within the ink composition during temperature increases which are normally encountered in thermal printing systems. The gas bubbles are comprised of gaseous materials which were previously dissolved in the ink compositions as discussed below. Typically, ink materials which are delivered using thermal inkjet technology are heated to an average temperature of about 25xc2x0 to 80xc2x0 C., depending on the printing system being used, by the die, or substrate, on which the heating resistor is formed. At temperatures within this range, the solubility of any air or other gases dissolved within the ink composition will decrease substantially. This condition (which is especially true in connection with water-based inks designed for plain-paper printing) causes super-saturation of the ink composition with the dissolved gases. In turn, the equilibrium kinetics associated with super-saturation will cause gas bubbles to form in the ink.
Gas bubbles can form in the pen structure, preventing ink from reaching the firing chamber. Consequently, the bubbles choke off the pen, and the pen ceases to jet ink onto a print medium. One can use higher organic inks to deal with outgassing. This approach was described in U.S. Pat. No. 5,700,315, entitled xe2x80x9cAnti-Outgassing Ink Composition and Method for Using the Samexe2x80x9d and issued to Donald E. Wenzel on Dec. 23, 1997. But there are some negative consequences to the straightforward use of the listed high organic solvents. Increased viscosity of the resulting ink leads to lower reliability, typically in short term decap and the firing frequency response of the pen. It also often leads to print quality defects on plain paper, such as poor edge acuity, strike-through, and increased dry-time.
More recent advances in inkjet printing have used underprinting with an ink or a fifth fluid, also called a fixer. Underprinting immobilizes the colorant, improving edge acuity, strike-through, and dry-time. However, a potential disadvantage of underprinting is increased paper cockle, due to the increased amount of water deposited on the print medium.
Thus, there is a need for an ink that, in combination with a fixer, re-tains high edge acuity and optical density while avoiding outgassing of the ink from a heated die in the pen. Such a combination should also evidence decreased paper cockle and curl.
In accordance with the present invention, an inkjet ink is provided that has a greater organic content than prior art inks. However, it is not simply enough to increase the organic content; other factors must be considered as well. These include controlling viscosity and surface tension. An inkjet ink composition that evidences reduced outgassing and reduces paper cockle and curl comprises about 20 to 40 wt % of one or more organic co-solvents, has a viscosity of 10 cp or less, and has a surface tension of 40 dyne/cm or less.
Advantageously, using a higher solvent ink reduces cockle. Cockle is also reduced as the water content of the ink is reduced.
The present invention uses higher solvent inks in combination with underprinting. In underprinting, a fixer solution is first xe2x80x9cprintedxe2x80x9d on the print medium, followed by printing the inkjet ink thereon. There is a synergism to this combination. Together, weaknesses of both approaches are addressed. The advantages are a longer life pen and reduced cockle and curl with acceptable print quality. Current embodiments have not used higher solvent content inks in their underprinting solutions.